Defense Intelligence Agency investigators concluded more than three years ago that Philippine military attaches in the United States were under orders to direct harassment of U.S. opponents of President Ferdinand Marcos, according to government sources.

That information is a key element in a multi-agency investigation fueled by the shooting death here Wednesday of Oscar Salvatierra, Los Angeles bureau marketing director of the Philippine News and a Marcos opponent.

Current and former U.S. officials involved in probes of Philippine espionage in this country say the anonymous intimidation and subsequent slaying of Salvatierra matches what is known of pro-Marcos operations here.

However, the political motives of apparent harassment incidents have often been much better disguised, they said.

"Usually, they have been made to look like something else," said one former U.S. investigator who conducted a lengthy probe of attacks on Marcos opponents in the United States.

He said U.S. intelligence has concluded that Philippine military attaches have been under orders to use pro-Marcos Filipinos, even some known for criminal activities, to intimidate Marcos opponents.

News publisher Alex Esclamado said today that Marcos emissaries have offered as much as $12 million to buy the 77,000-circulation English-language newspaper and stop its weekly blasts at the Philippine government.

He said he is a former Marcos supporter but is convinced that Salvatierra's slaying was a warning that he also would be killed if he did not stop his anti-Marcos activities.

Police in this affluent suburb 10 miles north of Los Angeles joined their Los Angeles counterparts and the FBI in continuing the investigation of the slaying of Salvatierra in his bedroom Wednesday morning. An autopsy today determined that he died of gunshot wounds to the head.

Some police sources and Filipino community leaders indicated continuing puzzlement about why Salvatierra, a relatively quiet opponent of the Marcos government who did not write for the newspaper, would be the target of a political murder. Many more outspoken Filipinos live in southern California.

Investigators indicated that they are checking business dealings of the 41-year-old accountant to see if other motives are possible.

White House spokesman Edward Djerejian, traveling with President Reagan on Air Force One to Grenada today, said of the shooting, "We consider this to be shocking, and we are investigating it vigorously."

In Manila, Corazon Aquino said in a statement that she is "deeply saddened" by the slaying. Aquino was declared the loser to Marcos in this month's disputed Philippine presidential election.

Many leaders of the anti-Marcos movement and some members of Congress have said they believe, because of death-threat letters received by Salvatierra and another News executive the day before the incident, that Marcos arranged the shooting of Salvatierra.

Rep. Stephen J. Solarz (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asian and Pacific affairs and a frequent Marcos critic, said he is "outraged" by the shooting and believes that it strongly indicates that "Marcos' death-squad tactics have been brought from the Philippines to the United States."

Solarz noted that a provision added to the arms export control act in 1981 prohibits arms sales to any nation whose government has been "determined by the president to be engaged in a consistent pattern of acts of intimidation or harassment directed at individuals in the United States."

For decades, conversations in the U.S. community of nearly 1 million Filipinos have been rife with tales of Marcos hit squads roaming the country.

The 1981 slaying in Seattle of Filipino union workers Silme Domingo and Gene Viernes and the 1977 disappearance of Marcos aide-turned-critic Primitivo Mijares have been blamed on Philippine espionage, although little or no evidence has been found to justify the claim.

Prosecutors who convicted three men in the Seattle killings blamed gambling disputes and unrest about changes the two victims were making in a union.

A Philippine News staff member said police were guarding the newspaper's Los Angeles bureau office just northwest of downtown and protecting senior account executive Stan Aragon, who also received a threatening letter Tuesday.

San Francisco Mayor Dianne Feinstein, calling the slaying "the direct result of what's going on in the Philippines," said police were guarding two newspaper staffers who also received threats.

University of Cincinnati law professor Michael Glennon, who headed a survey of Filipino espionage in 1979 as legal counsel to the Senate Foreign Relations Committee, said "the U.S. government was fully aware" of Philippine surveillance of Marcos opponents at that time but made no effort to stop it. He said he could not discuss classified information, but suggested that agents of the Taiwan government were followed more closely because they were thought to pose a greater threat to U.S. security.

Salvatierra lived in Glendale with his wife, mother and four children.